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SubjectRe: [patch v4] mm, oom: fix unnecessary killing of additional processes
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On 2018/07/21 5:14, David Rientjes wrote:
> diff --git a/mm/mmap.c b/mm/mmap.c
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3066,25 +3066,27 @@ void exit_mmap(struct mm_struct *mm)
> if (unlikely(mm_is_oom_victim(mm))) {
> /*
> * Manually reap the mm to free as much memory as possible.
> - * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard
> - * this mm from further consideration. Taking mm->mmap_sem for
> - * write after setting MMF_OOM_SKIP will guarantee that the oom
> - * reaper will not run on this mm again after mmap_sem is
> - * dropped.
> - *
> * Nothing can be holding mm->mmap_sem here and the above call
> * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
> * __oom_reap_task_mm() will not block.
> *
> + * This sets MMF_UNSTABLE to avoid racing with the oom reaper.
> * This needs to be done before calling munlock_vma_pages_all(),
> * which clears VM_LOCKED, otherwise the oom reaper cannot
> - * reliably test it.
> + * reliably test for it. If the oom reaper races with
> + * munlock_vma_pages_all(), this can result in a kernel oops if
> + * a pmd is zapped, for example, after follow_page_mask() has
> + * checked pmd_none().
> */
> mutex_lock(&oom_lock);
> __oom_reap_task_mm(mm);
> mutex_unlock(&oom_lock);

I don't like holding oom_lock for full teardown of an mm, for an OOM victim's mm
might have multiple TB memory which could take long time.

Of course, forcing someone who triggered __mmput() to pay the full cost is
not nice though, for it even can be a /proc/$pid/ reader, can't it?

>
> - set_bit(MMF_OOM_SKIP, &mm->flags);
> + /*
> + * Taking mm->mmap_sem for write after setting MMF_UNSTABLE will
> + * guarantee that the oom reaper will not run on this mm again
> + * after mmap_sem is dropped.
> + */
> down_write(&mm->mmap_sem);
> up_write(&mm->mmap_sem);
> }



> -#define MAX_OOM_REAP_RETRIES 10
> static void oom_reap_task(struct task_struct *tsk)
> {
> - int attempts = 0;
> struct mm_struct *mm = tsk->signal->oom_mm;
>
> - /* Retry the down_read_trylock(mmap_sem) a few times */
> - while (attempts++ < MAX_OOM_REAP_RETRIES && !oom_reap_task_mm(tsk, mm))
> - schedule_timeout_idle(HZ/10);
> + /*
> + * If this mm has either been fully unmapped, or the oom reaper has
> + * given up on it, nothing left to do except drop the refcount.
> + */
> + if (test_bit(MMF_OOM_SKIP, &mm->flags))
> + goto drop;
>
> - if (attempts <= MAX_OOM_REAP_RETRIES ||
> - test_bit(MMF_OOM_SKIP, &mm->flags))
> - goto done;
> + /*
> + * If this mm has already been reaped, doing so again will not likely
> + * free additional memory.
> + */
> + if (!test_bit(MMF_UNSTABLE, &mm->flags))
> + oom_reap_task_mm(tsk, mm);

This is still wrong. If preempted immediately after set_bit(MMF_UNSTABLE, &mm->flags) from
__oom_reap_task_mm() from exit_mmap(), oom_reap_task() can give up before reclaiming any memory.
test_bit(MMF_UNSTABLE, &mm->flags) has to be done under oom_lock serialization, and
I don't like holding oom_lock while calling __oom_reap_task_mm(mm).

>
> - pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
> - task_pid_nr(tsk), tsk->comm);
> - debug_show_all_locks();
> + if (time_after_eq(jiffies, mm->oom_free_expire)) {
> + if (!test_bit(MMF_OOM_SKIP, &mm->flags)) {
> + pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
> + task_pid_nr(tsk), tsk->comm);
> + debug_show_all_locks();
>
> -done:
> - tsk->oom_reaper_list = NULL;
> + /*
> + * Reaping has failed for the timeout period, so give up
> + * and allow additional processes to be oom killed.
> + */
> + set_bit(MMF_OOM_SKIP, &mm->flags);
> + }
> + goto drop;
> + }



> @@ -645,25 +657,60 @@ static int oom_reaper(void *unused)
> return 0;
> }
>
> +/*
> + * Millisecs to wait for an oom mm to free memory before selecting another
> + * victim.
> + */
> +static u64 oom_free_timeout_ms = 1000;
> static void wake_oom_reaper(struct task_struct *tsk)
> {
> - /* tsk is already queued? */
> - if (tsk == oom_reaper_list || tsk->oom_reaper_list)
> + unsigned long expire = jiffies + msecs_to_jiffies(oom_free_timeout_ms);
> +
> + if (!expire)
> + expire++;
> + /*
> + * Set the reap timeout; if it's already set, the mm is enqueued and
> + * this tsk can be ignored.
> + */
> + if (cmpxchg(&tsk->signal->oom_mm->oom_free_expire, 0UL, expire))
> return;

We don't need this if we do from mark_oom_victim() like my series does.

>
> get_task_struct(tsk);
>
> spin_lock(&oom_reaper_lock);
> - tsk->oom_reaper_list = oom_reaper_list;
> - oom_reaper_list = tsk;
> + list_add(&tsk->oom_reap_list, &oom_reaper_list);
> spin_unlock(&oom_reaper_lock);
> trace_wake_reaper(tsk->pid);
> wake_up(&oom_reaper_wait);
> }

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